This invention relates to a negative pressure type brake hydraulic pressure generating device which makes it possible to set a stroke on the operating side independently of a stroke on the output side (affecting the fluid amount consumed in the brake circuit), and particularly to a negative pressure type brake hydraulic pressure generating device which achieves good brake feeling.
A widely used conventional brake hydraulic pressure generating device is structured such that the brake operating force is amplified by means of a booster, and the amplified force is applied to a master cylinder. In this type of device, since the operating stroke of the brake pedal (hereinafter referred to as pedal stroke) directly corresponds to the amount of fluid discharged from the master cylinder, when an additional hydraulic device such as antilock system is activated, its influence inevitably reflects the pedal stroke.
For example, JP patent publication 2002-173016 proposes a brake hydraulic pressure generating device which copes with this problem. In the device, the interior of a fixed shell is partitioned into a constant pressure chamber and a variable pressure chamber by an axially slidable power plate (power piston). Inside the power plate, a piston is provided so as to be axially movable relative to the power plate, biased in the retracting direction by a spring (that is, stroke-power converter) for setting the stroke on the operating side. Also, inside this piston, which has its rear portion protruding outwardly from the fixed shell, a control valve and an input shaft are provided.
The constant pressure chamber is connected to a negative pressure source such as the intake manifold of the engine. Also, the variable pressure chamber communicates with the constant pressure chamber during non-operation of the brakes, and communicates with the atmosphere during operation of the brakes. Communication is changed over by a control valve built in the piston.
When the brake pedal is stepped in, the input shaft, which receives the pedal operating force, is pushed in. With relative movement between the piston and the input shaft at this time, the control valve first shuts off communication between the variable pressure chamber and the constant pressure chamber, and then brings the variable pressure chamber into communication with the atmosphere. Thus, the atmospheric air which depends on the brake operating amount flows into the variable pressure chamber, creating a pressure difference between the constant pressure chamber and the variable pressure chamber. Under this pressure difference, the power plate advances, applying its force to the master cylinder. At this time, the piston, too, advances under the differential pressure between the constant pressure chamber and the variable pressure chamber to a position where the pressure difference balances with the force of the spring. The piston stroke at this time is substantially equal to the stroke of the input shaft. The reaction force upon operation of the pedal is generated when the pressure in the master cylinder is applied to the tip of the input shaft. The pressure in the master cylinder is generated depending on the pressure in the variable pressure chamber.
In the thus structured device of the patent publication, since the power plate and the piston are combined so as to be axially movable relative to each other, it is possible to set the pedal stroke irrespective of the consumed fluid amount of the brakes.
With a brake hydraulic pressure generating device in which a piston having a built-in control valve for introducing atmospheric air and a power plate are combined so as to be axially movable to each other, the sliding resistance by an airtight seal portion provided between the piston and the power plate imparts a bad influence on the movement of the piston. That is, due to this slide resistance, during step-in of the brake pedal, the power plate drags the piston. During return of the brake pedal, the power plate tends to forcibly push back the piston. Thus, the stroke hysteresis of the piston increases, so that the brake feeling worsens.
An object of this invention is to suppress unintended movements of the piston resulting from the slide resistance between the piston and the power plate, thereby improving the brake feeling.